RFC: Move std::net::IpAddr types into core:.net.

text/0000-core-net-ipaddr-types.md

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+- Feature Name: `core_net_ipaddr_types`
+- Start Date: 2019-12-06
+- RFC PR: [rust-lang/rfcs#2832](https://github.com/rust-lang/rfcs/pull/2832)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Make the `IpAddr`, `Ipv4Addr` and `Ipv6Addr` types available in `no_std`
+contexts by moving them into a `core::net` module.
+
+# Motivation
+[motivation]: #motivation
+
+The motivation here is to provide common types for both `no_std` and `std`
+targets which in turn will ease the creation of libraries based around IP
+addresses. Embedded IoT development is one area where this will be beneficial.
+IP addresses are portable across all platforms and have no external
+dependencies which is in line with the definition of the core library.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+The `core::net::IpAddr`, `core::net::Ipv4Addr` and `core::net::Ipv6Addr` types
+are available in `no_std` contexts.
+
+Library developers should use `core::net` to implement abstractions in order
+for them to work in `no_std` contexts as well.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Currently, the IP address types depend on their corresponding `libc`
+counterpart for their `inner` value.
+
+IPv4 addresses are well-defined. [IETF RFC 791] specifically states:
+
+> Addresses are fixed length of four octets (32 bits).
+
+IPv6 addresses are well-defined. [IETF RFC 4291] specifically states:
+
+> IPv6 addresses are 128-bit identifiers
+
+Since the size and representation of IPv4 and IPv6 addresses are well defined,
+we can replace the `inner` value of `Ipv4Addr` with a `[u8; 4]` and the `inner`
+value of `IPv6Addr` with a `[u8; 16]`.
+
+The inner types `[u8; 4]` and `[u8; 16]` are expected to correspond to `u32`
+and `u128`  in big-endian byte order. Currently, this is already ensured:
+
+- `u32::to_be` is used when constructing the corresponding `libc` type for
+  `Ipv4Addr`.
+- The corresponding `libc` type for `IPv6Addr` is already represented as a
+  `[u8; 16]` internally on all platforms.
+
+[IETF RFC 791]: https://tools.ietf.org/html/rfc791
+[IETF RFC 4291]: https://tools.ietf.org/html/rfc4291
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Moving the `std::net` types to `core::net` makes the core library less *minimal*.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+- Given the size of IP addresses is well defined by IETF RFCs, there is no
+  inherent need to have these types depend on `libc`.
+
+- Eliminates the need to use different abstractions for `no_std` and `std`.
+
+- Alternatively, move these types into a library other than `core`, so they
+  can be used without `std`.
+
+# Prior art
+[prior-art]: #prior-art
+
+There was a prior discussion at
+
+https://internals.rust-lang.org/t/std-ipv4addr-in-core/11247/15
+
+and an experimental branch from [@Nemo157](https://github.com/Nemo157) at
+
+https://github.com/Nemo157/rust/tree/core-ip
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+None.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+`SocketAddr`, `SocketAddrV4` and `SocketAddrV6` could also be moved in the
+future.